Television receiver



Patented Nov. 4, 1941 2,261,795 TELEVISION RECEIVER Kenneth A. Chittick, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 26, 1938, Serial No. 192,716

Claims.

My invention relatesv to high voltage apparatus and particularly to television receivers of the type utilizing cathode ray tubes.

It has been found that in a television receiver the chassis and other metal parts which are ungrounded may acquire a high voltage charge due to leakage capacity and resistance between the highvoltage supply units and the chassis. Because of this charge, the operator of the receiver may receive a shock by coming into contact with thechassis, a control shaft or the like. If the chassis were grounded, the possibility of shock would be avoided but in many cases it is not con- ,venient to connect the chassis to ground.

It is accordingly an object of my invention to eliminate the above-described objectionable feature in television receivers or other high voltage apparatus in which the chassis or the like is not connected directly to ground.

v I Afurther object of my invention is to provide an improved television receiver.

Aiurther object of my invention is to provide an improved method of and means for removing an electrical charge from a television receiver chassis. v Other objects, features and advantages of my invention will appear from the following description, taken in connection with the accompanying drawing in which-- Figure l is a circuit diagram of one embodiment of my invention, and

Figure 2 is a view of a modification of tHe circuit' shown in Fig. 1.

Referring to Fig. 1 my invention is shown applied to va television receiver of the type having automatic background control. The receiver comprises video amplifier tubes I and 2 and a cathode ray tube 3. The tube I may be of the screenv grid type having an indirectly heated cathode 4, a control grid 6, a screen grid I and a plate 8. r f

Thetube 2 preferably is a combination triodediode having an indirectly heated cathode 9, a control grid II, a. plate I2 and a diode plate I3. I The cathoderay tube may be of the electrostatic focusing type having an indirectly heated cathode I4, a control electrode I6, a screen grid IT, a first anode I8 and a second anode I9. The tubeis provided with the usual fluorescent screen 2 I and the cathode ray may be magnetically deflected-by means of deflecting coils, not shown.

The receiver, chassis is indicated at 22, this chassis not being connected directly to ground.

The amplifier tube I is supplied with plate voltage through a plate resistor 23 and the usual peaking coil 24 for holding up the high frequency response. This plate voltage is supplied from the voltage divider 26 of a comparatively low voltage rectifier-filter unit, not shown. The connectionirom the plate circuit'terminal marked +275 v. to the upper end of the voltage divider 26 is omitted in order to simplify the drawing.

The output circuit of tube I includes a potentiometer comprising resistors 21 and 28 and con denser 29 from which the video signal is supplied to the input circuit of the triode section of the tube 2. This input circuit includes a grid condenser and a grid leak resistor 30.

An operating voltage is supplied to the ,plate I2 of the tube 2 through a plate resistor 3I and a peaking coil 32 from a point on the voltage divider 26.

The output of amplifier 2 is supplied to the control electrode I6 of the cathode ray tube through a direct current connection 33 whereby any direct current variations occurring in the plate circuit 3I-32 are impressed upon control electrode It. This permits the use of automatic background control.

Automatic background control is obtained by giving the grid condenser 25 and the grid leak resistor such values that, by grid leak biasing action, the synchronizing impulses make the bias on the grid I I vary in accordance with the picture background. This automatic background control circuit is described and claimed in application Serial No. 718,192, filed March 30, 1934, in the name of Waldemar J. Pooh and assigned to the Radio Corporation of America.

Considering now the cathode connections of tubes I,- 2 and 3, the cathodes of tubes I and 2 and the lower end of voltage divider 26 are connected to the chassis 2. The cathode of tube 3 is connected through a conductor 34 to the negative end of a voltage divider 36 in the high voltage supply unit 31. a

In order to protect the cathode ray tube at the time power is first-applied to the receiver, the connection from the cathode ray tube cathode I4 to the cathode 9 of the tube 2 includes a diode comprising the diode plate I3 and the cathode 9. This connection is provided as follows:

The diode plate I3 is connected to the upper I end of voltage divider 26 through resistors 38, 39 and M. The cathode I4 is connected to the resistor 39 through a conductor 42 and a variable tap 43. It will be seen that before the cathode 9 becomes heated to produce a voltage drop in plate resistor 3 I, the cathode I4 is positive with respect to the control electrode I6. In the specific example shown, cathode I4 is at 275 volts and electrode I6 is at 240 volts whereby the electrode I6 is 35 volts negative to bias tube 3 substantially to cut-01f. This protective circuit is described in Holmes Patent 2,092,373, issued Sep tember 7, 1937, and assigned to the Radio Corporation of America.

By adjusting the variable tap 43, the picture background may be controlled manually, this control being in addition to the automatic control.

The high voltages required by the cathode ray tube are supplied by the rectifier-filter unit 3'! which includes a rectifier tube 44 supplied from a transformer 46 having a primary winding 41 and secondary windings 48 and 49. The rectifier output is filtered by means of a series resistance and shunt condensers 52 and 53 and then supplied to the voltage divider 36.

The screen grid H, the first anode l8 and the second anode l9 are supplied with voltage from voltage divider 36 through conductors 54, 56 and 51, respectively. It will be noted that regardless of any variations in the voltage supplied by power unit 31, the ratio of voltages supplied to the cathode ray tube remains constant whereby the cathode ray beam remains focused.

The various parts of the voltage supply unit 3'! have leakage capacities and resistances to the chassis, these capacities and resistances at the alternating current end of the unit being represented at 58 and 59 and at 6i and 62. At the direct current end of the unit, the leakage capacities and resistances are represented at 63 and 64 and at 66 and 61.

It will be apparent that these leakage impedances will cause the chassis to acquire a potential intermediate the potential of the positive and negative sides or terminals of the unit 31. For example, if impedances 58 and 59 are equal to impedances 6| and 62, respectively, the chassis 22 will acquire an A. C. voltage intermediate that of the terminals of transformer winding 48 and will be at a high potential, namely, at a potential equal to one-half the voltage across winding 48.

This chassis potential in certain receivers will be about 3000 volts, for example. Because of the low current passed by the leakages, this probably is not a dangerous voltage but it will give an operator an unpleasant shock.

Likewise, if impedances 63 and 64 are equal to 66 and 61 respectively, the chassis 22 will acquire a D. C. potential which is equal to onehalf the voltage across the voltage divider 36.

In accordance with a preferred embodiment of my invention I connect the negative side of the unit 31 to the chassis 22 through a resistor R1 and a condenser C1, the impedances of R1 and C1 being low compared with the D. C. and A. C. leakage impedances, respectively, whereby there is a comparatively small potential dilference between the chassis and the negative or low voltage side of unit 31.

The chassis in turn is held at a comparatively low potential with respect to ground by means of a comparatively low impedance resistor R2 which is connected between chassis 22 and the grounded primary winding 41. It will be understood that winding 41 is grounded when the receiver is plugged into the power supply since one side of the usual lighting or power circuit is grounded as indicated at 50.

It should be noted that the low voltage side of unit 37 cannot be connected directly to chassis 22 (i. e., it must be connected through impedance units such as C1 and R1) because such a direct connection would short circuit the resistors 38 and 39 and the diode unit 9l3.

A direct connection in place of R2 would, of course, require that care be taken in plugging the receiver into the power line in the event that the chassis had been grounded. Otherwise, the plug might be inserted in the power supply socket with the wrong polarity and the line short circuited.

It will be understood that the several impedance and voltage values have been given merely by way of example. The important feature is that condenser C1 and resistor R1 have low impedances compared with the A. C. and D. C. leakage impedances, respectively. In the example given, condenser C1 has an impedance of about 13,000 ohms at cycles per second while the leakage impedance of 58 and 59 in parallel may be of the order of 6 megohms at this frequency. Assuming that impedances 6| and 62 are of the same value as 58 and 59 and that there is 6000 volts across secondary 48, the A. C. voltage difference between the chassis and low Voltage side of unit 31 will be about 3,000 volts=6 volts.

Likewise, considering the D. C. leakage, the impedance of impedances 63 and 64 in parallel may be about 50 megohms. Assuming impedances 66 and 6'! are of this same value, the potential diiference between the chassis 22 and the low voltage side of unit 31 will be about 1 megohm/50 megohm 3000 volts=60 volts.

That resistor R2 brings the chassis 22, and therefore the low potential side of unit 31 comparatively close to ground potential will be seen from the following discussion.

Because of leakage impedance between the primary winding 41 and the secondary winding 48, the mid-points of the two windings are at substantially the same potential. Therefore, without the resistor R2, the low side of unit 31 as well as the chassis 22 (assuming C1 and R1 are in the circuit) is about 3000 volts below ground potential.

This leakage impedance between windings is high for direct current, about 50 megohms, for example. By inserting the resistor R2 which is of comparatively low impedance, the chassis 22 is brought close to ground potential. In the example assumed, the chassis Voltage below ground is about 1 megohm/50 megohms 3000 volts or 60 volts.

The A. C. leakage impedance between windings is lower than the D. C. leakage impedance, about 10 megohms, for example. Thus, the A. C. voltage between the chassis and ground may be 1 megohm/ 10 megohms 3000 volts or 300 volts. However, this is a comparatively low voltage and at the low currents involved will not shock an operator.

A slight variation of my invention is shown in Fig. 2, where parts similar to those in Fig. 1 are indicated by like reference numerals. In this embodiment the receiver may be the same as shown in Fig. 1 but the primary circuit of the transformer 46 is provided with a filter for filtering out noise coming in on the line. The filter comprises condensers H and 12 of equal capacity having their junction point connected to ground. The filter further comprises a resistor 13 across the line which has its mid-point connected to the chassis 22. It will be seen that the chassis is connected to ground through an impedance of one megohm as in the case of the circuit shown in Fig. 1. The connections for maintaining the chassis close to the potential of the low side of the unit 31 are the same as previously described.

I claim as my invention:

1. A television receiver comprising a metal chassis having a high voltage unit mounted thereon, said voltage unit comprising a step-up transformer having a primary and a secondary and further comprising a rectifier and a filter connected to said secondary, said voltage unit having D. C. leakage impedances to said chassis, a resistor connecting the low voltage side of said voltage unit to said chassis, said resistor having low impedance compared with said leakage impedances, and a resistor connected between said chassis and said primary, said last resistor having an impedance which is low compared with the leakage impedances between said primary and said secondary.

2. A television receiver comprising a chassis of conducting material having a high voltage supply ,unit mounted thereon, said unit comprising a step-up transformer having a primary and a secondary which have leakageimpedances therebetween and further comprising a rectifier and filter connected to said secondary, said unit hava step-up transformer having a primary and a secondary which have leakage impedances therebetween and further comprising a rectifier and a filter connected to said secondary, said unit having A. C. and D. C. leakage'impedances to said chassis, a condenser and a resistor in parallel connected between the low voltage side of said unit and said-chassis, said condenser and resistor having low impedances compared with said A. C. and D. C. leakage impedances,respectively, and a resistor connected between said chassis and said primary, said last resistor having a low impedance compared with the leakage impedances between said primary and said secondary.

4. A television receiver comprising a chassis of conducting material having a high voltage supply unit mounted thereon, said receiver being of the type having automatic background control whereby the low voltage side of said unit cannot be connected directly to said chassis, said unit comprising a step-up transformer having a primary and a secondary which have leakage impedances therebetween and further comprising a rectifier and a filter connected to said secondary, said unit having A. C. and D. C. leakage impedances to said chassis, a condenser and a resistor in parallel connected between the low voltage side of said unit and said chassis, said condenser and resistor having low impedances compared with said A. C. and D. C. leakage impedances, respectively, and a resistor connected between said chassis and said primary, said last resistor having a low impedance compared with the leakage impedances between said primary and said secondary.

5. A television receiver comprising a chassis of conducting material having a high voltage supply unit mounted thereon, said receiver having a last stage video amplifier tube and a cathode receiver tube and being of the type having an automatic background control circuit in which there is a direct current connection between the plate of said video amplifier and the control electrode of said cathode ray tube, whereby the low voltage side of said voltage supply unit cannot be connected directly to said chassis, said unit comprising a step-up transformer having a primary and a secondary which have leakage impedances therebetween and further comprising a rectifier and a filter connected to said secondary, said unit having leakage impedances to said chassis, an impedance unit connected between the low voltage side of said voltage supply unit and said chassis, said impedance unit having a low impedance compared with said leakage impedances,

and an impedance unit connected between said chassis and said primary. said last impedance unit having a low impedance compared with the leakage impedances between said primary and said secondary.

KENNETH A. CHITTICK. 

